This invention relates to an improved apparatus and method for determining signal parameters such as the pulse width repetition ratio and the phase angle of periodic square wave signals.
German Pat. No. 11 79 634 discloses an apparatus and process for detecting and determining the phase angle between two alternating currents or alternating voltages. In the disclosed apparatus two square wave signals, each having equal pulse width repetition ratios, are applied to a logical gate circuit which acts to produce at its ouputs a square wave signal. The time averaged value of this square wave signal is proportional to the magnitude of the phase angle. A measuring instrument of the type which is sensitive to current polarity and has a scale with the zero position at the center of the scale can be coupled to the outputs of the gate circuit so that the magnitude and polarity of the phase angle can be displayed directly. In this prior art approach, an indicator such as a pointer is used to provide an analog display of the phase angle. Such an analog display is limited in accuracy; moreover, reading inaccuracies are increased at low frequencies of the square wave signals due to hunting and oscillation of the indicator. In addition, the voltage levels of the two output gates of the gate circuit must correspond with one another closely in magnitude. In order to achieve this correspondence, it is customary to balance the gate circuit. This balancing is not only difficult and time-consuming to achieve, but it also must be repeated from time to time to insure the accuracy of the system.